1. Technical Field of the Invention
The present invention relates to asynchronous transfer mode (ATM) networks and, in particular, to variable bandwidth access to an ATM network for wireline and wireless communications.
2. Description of Related Art
Broadband transmission and switching have become important topics in the communications industry. A new system has been implemented for transmitting broadband and narrowband packet and circuit signals over a broadband network. This system is commonly referred to as asynchronous transfer mode (ATM). In ATM, data is packed into frames, with each frame segmented into a plurality of fixed length blocks called "cells." Each ATM cell is fifty-three bytes (or octets) long and consists of a five byte header indicating, as one of its functions, the destination of the cell, followed by a forty-eight byte payload containing the data to be transmitted to that destination.
The use of ATM cells permits the information transfer rate over the physical medium of the broadband network to adapt to actual service requirements. Depending on the capacity required, the number of cells per unit of time may be increased in a variable bit rate (VBR) ATM network up to the transmission bit rate limit of the physical medium. Alternatively, constant bit rates (CBR) are supported, with the transmission bit rate adjusted by the inclusion of idle or fill in cells when necessary. The cells are transported over the physical medium at a typical constant bit rate of, for example, 155.52 Mbits per second. Faster bit rates are supported for communications occurring solely within the ATM network.
One important characteristic of ATM technology relates to its protocol architecture and is built around the so-called "core-and-edge" principle. The protocol functions specific to the information type being transported, such as retransmissions, flow control, and delay equalization, are performed in user terminals at the "edges" of the ATM network. This leaves an efficient, service-independent "core" network, including only simple cell-transport and switching functions. A user-to-network interface (UNI) implementing an ATM adaptation layer (AAL) is provided at the "edge" of the network to connect the user terminals to the core network thereby allowing for service-independent ATM transport. The ATM adaptation layer performs a mapping operation between the bit stream data format of the user terminals external to the ATM network and the payload field of an ATM cell transmitted through the network.
A number of different bit stream data formats are known and used external to the ATM network. Most of these formats are user specific in that they support and accommodate only the data in the format and rate transmitted to or from user terminals. For example, in the telecommunications art, separate bit stream data formats are known for packet voice and pulse code modulated (PCM) signals. Furthermore, high quality video and data (file transfer) applications have still further different bit stream data formats. There would be a distinct advantage to having a single bit stream format capable of supporting variable bandwidth ATM network access with respect to a plurality of different user terminals and their associated data formats. Such a bit stream would preferably be flexible enough to convey packet and PCM voice, data and/or packet video, and further provide built-in capabilities for operation and maintenance and add/drop multiplexing.